Collection and Processing
The New York State Department of Environmental Conservation (NYSDEC) Division of Solid and Hazardous Waste Materials regulates landfill closure and post-closure operations under 6 NYCRR Part 360, Solid Waste Management Facilities (Title 6 of the Official Compilation of Codes, Rules, and Regulation of the State of New York). The Solid Waste Management regulations require the identification and control of current or future potential releases of pollutants and the control and mitigation of any potential impacts once landfill operations have ceased. Among the requirements, all of which are met at Fresh Kills, are landfill gas control, leachate collection and treatment, and a post-closure operation and maintenance plan.
Landfill Gas Collection System
Landfill gas is a mixture of methane, carbon dioxide, and trace amounts of non-methane organic compounds (NMOCs) created by anaerobic bacteria who feed on decomposing waste. The landfill cap prevents landfill gas from migrating into the atmosphere. A network of underground wells and pipes collect landfill gas from the capped landfill mounds and send it to an onsite purification plant. The infrastructure involved in this process is known as the landfill gas collection system.
Collection begins with vertical landfill gas wells that extend down into the waste layer. The bottom of each well is perforated to allow particles of landfill gas inside. The vertical wells are connected to a network of lateral pipes, which are connected to nearby flare stations. The flare station’s blowers create a vacuum that pulls landfill gas into the vertical wells and through the network of lateral pipes.
The three flare stations at Freshkills Park can be easily identified by their large white stacks. Before 1982, the landfill gas was burned before being released into the atmosphere. Combustion transforms the methane in landfill gas into carbon dioxide, which is less harmful to the atmosphere. Since 1982, the landfill gas has been treated and recycled instead of burned. Large transmission pipes at each flare station now transport the landfill gas to the onsite landfill gas purification plant.
At the landfill gas purification plant, the gas is refined into pipeline-grade natural gas through a series of physical and chemical reactions. When the landfill gas first arrives at the plant, it is compressed and cooled until any water vapor condenses. The condensate is collected and sent to the nearby leachate treatment plant. The compressed gas goes through a pre-treatment process that removes hydrogen sulfide, and then a treatment process to remove carbon dioxide. The City of New York then sells the purified natural gas to National Grid, who distributes the gas to Staten Island residents for cooking and heating fuel.
Over time, the volume of landfill gas and concentrations of methane in the gas decrease. The Landfill Gas Control System currently collects about 3 million cubic feet of landfill gas per day, purified to produce about 1.5 million cubic feet of methane. In the future, it could be possible to turn off the active gas collection system. When that happens, a passive venting system with gooseneck pipes would take over. This system would go into effect when it’s determined that the concentrations of methane in the landfill gas are safe to emit to the atmosphere and that the volume of landfill gas generated by the site is so small that the active extraction system is no longer necessary.
Leachate Collection System
Leachate is created when rainwater percolates through decomposing garbage and picks up particles, including potential contaminants, from the garbage along the way. The leachate collection system consists of cut-off walls, pipes, and pumps that collect the leachate and prevent it from migrating into the adjacent soils and surface water. The collected leachate is then pumped to a treatment plant that separates the water from the waste materials.
The naturally-occurring soil underlying the landfilled material on-site is made of a fine silt clay with low permeability. The clay has prevented the leachate from migrating into deeper layers of soils below the landfill. Containment walls made of a concrete-like material are buried deep underground around the outside edges of each mound to provide extra protection against leachate migrating vertically away from the mounds. On the interior side of the containment walls are trenches that contain perforated pipes. The pipes are approximately 2 feet below groundwater level, which allows the leachate to drain from the mounds towards the pipes. The piping is connected to pumping stations which in turn pump the leachate to the treatment plant.
At the treatment plant, the leachate undergoes a series of physical and chemical processes to separate harmful materials from clean water. The Leachate Treatment Plant was updated in 1994, and it currently treats about 300,000 gallons of leachate per day. First, the leachate is treated biologically with anaerobic bacteria that break down any remaining organic matter. Then, sodium hydroxide and aluminum sulfate are added to the leachate to transform any soluble metals into their non-soluble (solid) form. The mixture of solids and liquid is then sent to a clarifier and left to settle to allow collection and removal of the solids. The solids are dewatered and compressed into sludge cakes that can be transported to other facilities in New York City for further treatment. The remaining liquid is filtered through sand, which captures any remaining solids, before being discharged into the Arthur Kill. Clean water that is discharged to the Arthur Kill is tested to ensure that it meets the discharge limits set by New York State Department of Environmental Conservation.